Power supply is widely used in various types of electronic devices nowadays. As technology advances constantly, the output power of power supply also increases continuously. To further boost the output power, its power efficiency has to be enhanced. There are many factors impacting efficiency, such as the input power factor and loss of power conversion. Power factor correction (PFC in short hereinafter) converter is used to improve the input power factor in the power supply. An active PFC circuit can improve the power factor to 95% or more. Referring to FIG. 1, the input power goes through a rectification circuit 8 and is regulated by the PFC circuit 1 to correct the input power factor. Then the output voltage of the PFC circuit 1 is further regulated by a Power conversion unit 9 to become a constant output voltage. The Power conversion unit 9 may be a converter or a switch type conversion circuit. The PFC circuit 1 and the Power conversion unit 9 are respectively controlled by a first control unit 11 and a second control unit 91. A typical PFC circuit 1 includes a switch 12, an inductor 13, a storage capacitor 14, a diode 15 and the first control unit 11. The first control unit 11 receives a feedback signal from the output voltage of the PFC circuit 1 and incorporates with a zero current detection (ZCD) circuit and a RS flip-flop to generate a period control signal to drive the switch 12, thereby regulate the input power factor. The operation principle of the PFC circuit 1 is known in the art, thus details are omitted herein. To balance the pulsating input power and the constant output power, the storage capacitor 14 must have a greater capacitance and usually use electrolytic capacitor. The rated lifetime of conventional electrolytic capacitor is only a few thousand hours. So the conventional PFC circuit 1 has a limited lifetime due to the usage of electrolytic capacitor. Please refer to FIGS. 2 and 3, where vg is the voltage after rectification, vCB is the storage capacitor voltage in the PFC circuit, and vo is the output voltage of the power supply. The multiplication product of vin and iin. pin is the instantaneous input power (referring to FIG. 3). As shown in FIGS. 2 and 3, the input power pin is pulsating and the output power of PFC circuit is constant owing to its constant power load which is usually a DC/DC converter. The storage capacitor voltage vCB in the conventional PFC circuit is constant with small voltage ripple, saying about 10˜20V peak to peak. Therefore, electrolytic capacitor which has large capacitance is required. Therefore, incorporating the PFC circuit 1 in a long life electronic element results in a waste. For instance, adopting the conventional PFC circuit 1 in a driving circuit to drive LEDs whose lifetime is about 100,000 hours, the PFC circuit 1 could last only up to about 20 thousand hours. Then the circuit board with the PFC circuit 1 mounted thereon and LEDs soldered thereon have to be replaced. The LEDs which are being used for less than one half of their lifetime also have to be discarded with the entire circuit board. This is a waste and makes the cost higher. Therefore, the lifetime of the PFC circuit 1 must be prolonged to deserve long life LEDs.